When iron ores are roasted at temperatures above about 1500.degree. F., the magnetite mineral contained in the ore oxidizes rapidly enough to act as a significant source of heat for the process. The fuel value of magnetite burned in this way is about 7000 BTU/lb. When magnetite is burned, hematite is produced.
Hematite, naturally-occurring or produced from magnetite, can be reduced to artificial magnetite, using hot carbon monoxide as reducing agent. When conditions are properly controlled, a small amount of heat is generated in the conversion process.
Artificial magnetite can be burned by oxidation at low temperatures to produce magnetic gamma hematite. In this latter reaction, the exothermic heat produced is so substantial that the overall three-step process can be made self-sustaining.